I'm trying to size a natural gas forced air furnace and I know that the wrong size furnace can result in discomfort, harm to the appliance, and excessive fuel usage. I've spent some time online reading about heat loss calcs, and it seems that one of the most accurate ways to size a new system is by looking at the (edit: input/output of the existing furnace, the volume of gas used, and making a calculation based on that). There is also a natural gas H2O tank who's usage I would have to separate from the furnace usage (not sure how, but the water usage is metered and this may also help). Here's some basic info about the space, and thanks for any help:
1. Rochester/Buffalo NY area
2. 700 sq. ft second floor with 4" of fiberglass in walls and ceiling and no insulation in the floor (above a heated space).
3. 8 double pane vinyl windows (3' x 2.5')
4. Current heat source is a 40-50 year old natural gas furnace in the basement
5. The existing furnace has very large duct work coming from it.
6. There is also a natural gas hot water tank (may make it difficult to determine past fuel usage of heater only)